Modern power semiconductor chips, for example IGBTs, MOSFETs, JFETs, etc. attain very high junction temperatures in operation. To dissipate the resulting waste heat the power semiconductor chips are mounted on metallized ceramic substrates which may be bonded to a secure base plate of the power semiconductor module. Due to the high achievable chip temperatures the bonding layers between the power semiconductor chip and the ceramic substrate and between the ceramic substrate and the base plate are likewise exposed to very high temperatures. Since conventional solder bonds lack adequate long-term stability at such high temperatures, diffusion soldering is being developed or e.g. pressure sintering has already become established as a means for producing alternative bonding layers. Thus, for example, a power semiconductor chip may be bonded by means of a diffusion solder layer to the metallization topping the substrate whilst a pressure sintered bond may be used to bond the metallization bottoming the substrate. The top side of the semiconductor chip facing away from the ceramic substrate is contacted by means of a bond wire.
However, producing such an assembly is anything but trivial since the various bonding processes need to satisfy diverse requirements which have negative consequences for each other in part. For instance, producing a diffusion solder bond between the semiconductor chip and the substrate necessitates an adequate soldering temperature, as a result of which the top chip metallization and the surfaces of the substrate metallization located outside of the diffusion solder bond become oxidized with the problem that the bond to the top chip metallization and/or to the top substrate metallization lacks long-term stability due to oxide layers on the metallization, e.g. layers of copper oxide on copper metallizations to be bonded, negatively influencing the long-term stability of the bond.
Although, it is basically possible to avoid oxidation by soldering in a vacuum process chamber, producing a sinter bonding layer between a substrate and a base plate has to be done in an oxygen atmosphere, resulting in producing both bonds necessitating in any case a change from vacuum to a process atmosphere or vice-versa, adding to the time needed for the process, reducing the throughput and thus hiking the costs involved.
If, on the other hand, the pressure sintered bond between the substrate and the base plate were to be produced as the first of the bonds needed, oxidation of the top substrate metallization would occur during sintering in the zones in which the diffusion soldering between the power semiconductor chip and the top substrate metallization or in which a bond to the top substrate metallization is to happen with the drawback that oxide layers on a metallization to be soldered hamper diffusion of metal from the metallization into the molten solder, significantly lowering the quality of a diffusion solder bond to be produced. As already described, this applies correspondingly to a bond on an oxidized metal surface.
If, instead, the wire bonds were produced first, this would make it impossible to subsequently produce a pressure sintered bond between the substrate componented with the power semiconductor chip and the base plate, because this would require contact pressure being applied to the full surface of the power semiconductor chip and the substrate, which is prohibitive, however, when wire bonds already top the chip since they cannot withstand the pressure.